The colloquial use of "unique" is a quality, that it is special within its group and should be treated as special. One out of three is not very special, yet one out of a million is. Hence, while both are unique, the latter is more unique.

Another example: Someone may say "so-and-so is unique", just to be answered with, "but how unique is he?". The answer to that is a comparative value, which just may be "he is very unique".

Heh ... that sent me scurrying to my "Concise Oxford Dictionary". Even back when it was published (early nineties) it seems they were making an allowance for degrees of uniqueness:

2 disp. unusual, remarkable (the most unique man)

Just another example of how braindead usage can alter the meaning(s) of a word ... but at least it validates your assertion.

I'm reminded of the notion that some infinities are larger than others:
There's an infinite number of reals in the range 1..2.
There's an infinite number of reals in the range 1..3.
But there's clearly more reals in the range 1..3 than in the range 1..2 (because all of the reals in the latter also belong to the former, but not vice-versa). Therefore the infinite number of reals in the range 1..3 is greater than the infinite number of reals in the range 1..2.

If we can get people to start talking in terms of degrees of infiniteness then we'll eventually see that in the Oxford Dictionary, too, no doubt.

A degree of uniqueness is colloquial, but what about axes of uniqueness? A purple human might be unique as might a green one. A human with 21 toes would probably be unique. Surely a single ever green person otherwise normal would be as unique as before if along came a single ever purple human who also had alone in history 21 toes. These are notable differences from everyone else. In conversation, though, how incorrect would it be to call one more unique than the other?

Also, there is the idea of uniqueness within a given set. You might be the only computer programmer in your household. For that set of people in your household, that single trait could be unique. On Perlmonks, being a programmer is not unique. Your account here is unique, but the same username might be used on some other site you or by someone else. Yet nobody else in the world has all the same traits as you.